Dots of the same line did not stack back in classic. Best you could do was one drones line dot and one fire/debuff line dot. The highest level version overwriting what ever dot of the same line was already on the mob. If another character of higher level cast the same spell it would overwrite the lower level players dot as well.
Ex. lvl 59 casts winged death on Trakanon, then a level 60 would cast the same spell. The level 60's would overwrite the lvl 59 player, and the level 59 would not get their version to stack after that until the lvl 60's expired.
It was eventually changed some time after Velious (maybe Luclin) to allow additional dot stacks from different players. Ex. 2 winged deaths, 2 boil bloods.
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